Built-in self test for memory arrays using error correction coding

ABSTRACT

A memory self-testing system, apparatus, and method are provided which allow for testing for a plurality of bit errors and passing memory arrays having an error level which is correctable using selected error correction coding. An exemplary system embodiment includes a memory array, a comparator, an integrator, and a test control circuit. The memory array is adapted to store input test data and output stored test data during a plurality of memory read and write test operations. The comparator compares the input test data and the stored test data for a plurality of bit positions, and provides a corresponding error signal when the stored test data is not identical to the input test data for each bit position of the plurality of bit positions. The integrator receives the corresponding error signal and maintains the corresponding error signal for each bit position during the plurality of test operations. The test control circuit provides a fail signal when a predetermined level of corresponding error signals have been provided for the plurality of bit positions.

FIELD OF THE INVENTION

The present invention relates, in general, to integrated circuits havingdata storage capability and, in particular, to memory architectureshaving built-in self test capability which provide for detectingpredetermined error levels which are correctable using error correctioncoding.

BACKGROUND OF THE INVENTION

A semiconductor data storage device, such as a random access memory(“RAM”), static random access memory (“SRAM”), dynamic random accessmemory (“DRAM”), synchronous dynamic random access memory (“SDRAM”),magnetic random access memories (“MRAMs”), electronically erasableprogrammable read-only memory (EEPROM), and other forms of a memory, aredesigned to store data in a memory array of memory “cells”. Each memorycell, consisting of one or more transistors per bit of storage, isprogrammed during a write operation and is capable of maintaining thestored data, with or without memory refresh cycles, depending upon thetype of memory. The memories may be included in any type of integratedcircuit (“IC”), such as an IC that substantially contains only a memoryarray, as a dedicated memory IC, or included as an “embedded” memory aspart of an IC containing additional, other circuits, such as embeddedRAM, DRAM, SDRAM, etc., with a processor, digital signal processor,controller, wireless telecommunication IC or other communication IC.

A memory array is defined by a number of transistors, forming memorycells, generally arranged in a grid pattern having a plurality (orseries) of rows and columns. Rows are typically utilized to select oneor more data words, with the columns providing the input (for a memorywrite operation) and output (for a memory read operation) for each bitof each such word.

Memory arrays are tested in a wide variety of ways. External probe teststypically provide for continuity testing, and for testing of some AC andDC parameters. Many of these tests utilize one or more test patterns, inwhich test data is stored in the memory, read from the memory and thencompared with the original test data. Typical patterns, for example, areall 1's, all 0's, checkerboard, stripe, marching, galloping, slidingdiagonal, waling, and ping-pong, and test time varies with the selectedpattern. Different patterns are also operative to detect differentfaults and failure modes during testing, such as “stuck at” faults,pattern sensitivity (such as interference and bit-line imbalancefaults), multiple writing, refresh sensitivity, open and short circuits,leakage current faults, sense amplifier recovery, access time, voltagebump failures, decoder failures, and other faults and defects.

Self-testing, rather than external probe testing, has also beenincorporated into many memory ICs, through the addition of additionaltest circuitry within the IC. Prior art built-in self-testing, however,typically determines whether there are no faults, thereby “passing ” thememory as acceptable, or determining that there is at least one fault,and thereby “failing” the memory as unacceptable. Such prior artself-testing does not differentiate between memory ICs which have asingle memory defect, such as a defect affecting a single bit, andmultiple memory defects (affecting at least two bits), such as a defectaffecting a plurality of bits.

A representative, prior art memory self-testing circuit 10 isillustrated in FIG. 1. Input test data is compared, bit position by bitposition (i.e., by corresponding column), with the stored test data fromthe memory 20 under test, in comparator 50. In the event that the storeddata does not match the test data at a given bit position, an outputindicating such a failure (such as a logic 1 or high voltage state) isprovided on one of the corresponding comparator output lines 55. Priorart memory self-testing circuits (such as 10) are incapable ofdifferentiating multiple bit errors from a single bit error (such as oneerror versus multiple errors) because, following comparison of test datawith stored data, any indicator of failure for a bit position istypically “OR'd” (OR gate 60) with the indicators of all other bitpositions. If any one bit position has a failure, the output of the ORgate 60 will indicate a failure. The outcome from the OR gate 60 is thenlatched (with feedback) (latch 65), so that once a failure has occurred,it will not be overwritten, and the failure indicator 70 will indicate amemory failure. As a consequence of the OR process, the self-testingcircuit provides information that at least one error has occurred, butcannot determine that more than one error may have occurred.

Other types of memory testing are also utilized in the prior art, suchas testing which may be used with memory ICs having redundant or “spare”portions of the memory array. In such testing, memory ICs having noerrors may be fully passed, and memory ICs having one or more bit errorsmay be “conditionally passed”, in a first tier of testing. Those memoryICs which have been conditionally passed may then be physically repairedby disabling one or more rows or columns having defects, andsubstituting new rows or columns from the additional, redundant hardwareavailable in the memory IC, potentially followed by additional testing.This testing requires tracking of any defect of every cell of the array.Moreover, the ability to utilize or repair such conditionally passed ICsis not assured by a definitive count or other test control criteria.More particularly, memory ICs having up to a predetermined level of biterrors cannot be fully and automatically passed, as those memories mayor may not be repairable in fact. In addition, such testing requires theavailability of spare or redundant parts of a memory IC, increasingmemory size requirements without increasing the actual available memory.Such testing may also require several levels of testing, the initialconditional testing, followed by repair and potentially additionaltesting.

These current self-testing modes also do not utilize and take advantageof error correcting capabilities which may be available in the codingprocess for the data to be stored in the memory. Such error correctingcapabilities are typically utilized to correct for “soft” errors, suchas memory read errors which may occur due to external interference, suchas from cosmic rays, other forms of background radiation, or other noisesources. As a consequence, by only detecting that there is at least onebit error, prior art memory self-testing fails memory ICs having asingle defect, even though the single bit error could be correctedthrough error correction codes. By not differentiating between levels oramounts of potential defects, the prior art memory self-testing resultsin discarding memory ICs which are, in fact, capable of being utilizedwithin IC error specifications. Such prior art memory self-testing,therefore, decreases the available IC yield from IC manufacturing.

As a consequence, a need remains to provide a built-in self-testingcircuit for a memory array which can differentiate between levels oramounts of potential defects. Such a built in memory self-testingcircuit should provide for detecting bit errors, and for differentiatingmemory ICs having bit errors which are correctable within the capabilityof a selected error correction code from memory ICs having bit errorswhich are not correctable or which otherwise exceed the capability ofthe selected error correction code. Such a built in memory self-testingcircuit should not require additional, redundant memory, should notrequire defect tracking by specific memory cell, and should provide adefinitive rather than conditional test result.

SUMMARY OF THE INVENTION

The present invention provides an apparatus, system and method fortesting memory arrays. The various embodiments of the present inventionprovide built-in self-testing (“BIST”) for a memory array which candifferentiate between levels or amounts of defects or other faults, sothat those memory ICs having bit errors which are correctable within thecapability of a selected error correction code are differentiated fromthose memory ICs having bit errors which are not correctable or whichotherwise exceed the capability of the selected error correction code.The various embodiments of the present invention allow for an increasedand improved IC manufacturing yield, as memory ICs having correctablebit errors are not required to be discarded as failed ICs. In addition,the various embodiments of the present invention provide for a reducedtest time using less complicated testing patterns, while simultaneouslyproviding complete fault detection coverage.

Exemplary embodiments of the invention provide a memory self-testapparatus, in which the memory is adapted to store input test data andoutput stored test data during a plurality of memory read and write testoperations. The apparatus and system include a comparator, anintegrator, and a test control circuit. The comparator is adapted tocompare the input test data and the stored test data for a plurality ofbit positions, and to provide a corresponding error signal when thestored test data is not identical to the input test data for each bitposition of the plurality of bit positions. The integrator is coupled tothe comparator to receive the corresponding error signal, and theintegrator adapted to maintain the corresponding error signal for eachbit position during the plurality of test operations. The test controlcircuit is coupled to the integrator, the test control circuit adaptedto provide a fail signal when a predetermined level of correspondingerror signals have been provided for the plurality of bit positions.

An exemplary method of testing a memory array in accordance with theinvention is also provided. The method comprises comparing the inputtest data and the stored test data for a plurality of bit positions;providing a corresponding error signal when the stored test data is notidentical to the input test data for each bit position of the pluralityof bit positions; maintaining the corresponding error signal for eachbit position during the plurality of test operations; and providing afail signal when a predetermined level of corresponding error signalshave been provided for the plurality of bit positions.

Another exemplary system in accordance with the invention comprises amemory array, a test data generator, a comparator, an integrator, and atest control circuit. The memory array is adapted to store input testdata and output stored test data during a plurality of memory read andwrite test operations. The test data generator is adapted to generate,as input test data, a plurality of test patterns corresponding to theplurality of test operations, with each test pattern of the plurality oftest patterns adapted to detect single-bit errors. The comparator isadapted to compare the input test data and the stored test data for aplurality of bit positions, and to provide a corresponding error signalwhen the stored test data is not identical to the input test data foreach bit position of the plurality of bit positions, wherein theplurality of bit positions correspond to one or more data words, witheach data word having a plurality of data bits and a plurality of errorcorrection bits. The integrator is adapted to maintain the correspondingerror signal for each bit position during the plurality of testoperations. The test control circuit is adapted to provide a fail signalwhen a predetermined level of corresponding error signals have beenprovided for the plurality of bit positions, with the predeterminedlevel of corresponding error signals corresponding to an errorcorrection capability of a selected error correction code.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention and the embodiments thereof, from the claims and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will bemore readily appreciated upon reference to the following disclosure whenconsidered in conjunction with the accompanying drawings, wherein likereference numerals are used to identify identical components in thevarious diagrams, in which:

FIG. 1 (or FIG. 1) is a circuit diagram of a prior art memory self-testcircuit.

FIG. 2 (or FIG. 2) is a block diagram of an exemplary memory systemhaving built-in self test in accordance with the teachings of theinvention.

FIG. 3 (or FIG. 3) is a block diagram of an exemplary memory built-inself test circuit in accordance with the teachings of the invention.

FIG. 4 (or FIG. 4) is a flow diagram of an exemplary method of memorytesting in accordance with the teachings of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

While the present invention is susceptible of embodiment in manydifferent forms, there are shown in the drawings and will be describedherein in detail specific embodiments thereof, with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit theinvention to the specific embodiments illustrated.

As mentioned above, the various embodiments of the present inventionprovide built-in self-testing (“BIST”) for a memory array which candifferentiate between levels or amounts of defects or other faults. Thevarious embodiments of the built in memory self-testing of the presentinvention provide for detecting bit errors, and for differentiatingthose memory ICs having bit errors which are correctable within thecapability of a selected error correction code from those memory ICshaving bit errors which are not correctable or which otherwise exceedthe capability of the selected error correction code. As a consequence,utilization of the various embodiments of the present invention allowfor an increased and improved IC manufacturing yield, as memory ICshaving correctable bit errors are not required to be discarded as failedICs. In addition, the various embodiments of the present inventionprovide for a reduced test time using less complicated testing patterns,while simultaneously providing complete fault detection coverage.

FIG. 2 (or FIG. 2) is a block diagram of an exemplary memory system 100having built-in self test in accordance with the teachings of theinvention. Referring to FIG. 2, the system 100 includes a memory array110 having a plurality of cells 105, with each cell 105 coupled to acorresponding row 115 (for data word selection) and column 120 (bitline). The system 100 may utilize any type of memory array 100, such asRAM, SRAM, SDRAM, DRAM, MRAM, EEPROM, etc., and may be included withinany type of IC, such as a memory IC or embedded memory with othercircuitry.

Each cell 105 is generally activated for writing to or reading from thememory array 110 by selection of its corresponding row 115, withinformation (one or more bits) provided on the cell's one or morecorresponding columns 120 as either input data (for writing to memoryarray 110) or output data (for reading from memory array 110). A row 115is selected by provision of a row address to the row decoder 125, andsuch row 115 selection generally includes one or more data words withcorresponding error correction bits, depending upon the size of thememory array 110. For example, in an exemplary embodiment, selection ofa data word will generally include selection of 32 data bits and 6 errorcorrection code (“ECC”) bits, and a given row 115 generally includes aplurality of data words.

As the columns 120 corresponding to the bits of a plurality of datawords are generally spaced apart (interspersed or interleaved)throughout the memory array 110, selection of a particular data word(from a plurality of data words) from an activated row 115 isaccomplished by selection of particular columns (bit lines) 120, byprovision of a column address to a column decoder 130 (illustrated inconjunction with a sense amplifier and input/output gating 135). Data tobe stored in a selected row 115 and selected columns 120 (memory write)is provided, generally, through an input data buffer 140, such as inputtest data from test data generator 150. Such a test data generator 150may be included within the IC having the system 100, or may be off-chip.Stored data (such as stored test data) to be provided from a selectedrow 115 and selected columns 120 (memory read) is output through thesense amplifier (of 135) to the output data buffer 145. In the exemplaryembodiments, any error correction (150) is applied to stored and readdata after BIST testing (or independently of BIST testing, i.e., insubsequent, actual non-testing operation of the memory array 110 andsystem 100), in order to allow direct testing of the ECC bit positionsof the memory array 110. It will be apparent to those of skill in theart that numerous other equivalent variations are available and arewithin the scope of the present invention.

Input test data from the test data generator 150 is stored in the memoryarray 110 and, following a memory read operation, is provided as outputor stored test data to the self-test circuit 200 of the presentinvention. The same input test data from the test data generator 150 isalso provided directly to the self-test circuit 200, either directlyfrom the test data generator 150 or from the input data buffer 140.Following completion of testing, as discussed in greater detail below, apass or fail signal is provided from the self-test circuit 200 to thepass/fail indicator 230, to indicate whether the system 100 isacceptable or unacceptable.

FIG. 3 (or FIG. 3) is a block diagram of an exemplary memory built-inself test circuit 200 in accordance with the teachings of the invention.The stored (or output) test data read from the memory array 110 andprovided to the self-test circuit 200 is compared, bit position by bitposition, with the input test data (provided directly to the self-testcircuit 200), by comparator 205, for all test data stored in the memoryarray 110 (i.e., row by row (all data words) and column by column (allbit positions)). As illustrated, comparator 205 compares input test datawith stored test data for “N” bit positions, illustrated as bitpositions 0 (zero) through N-1. Depending on the selected embodiment, Nmay correspond to the number of columns of the array or the number ofbits in a data word, for example.

The input test data and stored test data are compared, for each bitposition, and if they do not match (are not identical), a bit error hasoccurred and is reflected in the state of the bit (e.g., logic 1 orlogic 0) output on the corresponding output line 235 of comparator 205.The existence of any error (discrepancy) is maintained, separately foreach bit position (which corresponds to a column 120 of the memory array110), by integrator 210. Using integrator 210, once an error hasoccurred for a given bit position as determined by comparator 205, thaterror information is maintained, even if other bits from other previousor subsequent data words are correct. In an exemplary embodiment, theintegrator 210 is implemented using a plurality of latch circuits 215,with each bit position having a separate latch 215, and with each latchhaving internal feedback (as illustrated) to maintain any errorinformation through repeated testing cycles. For example, if an errorhas occurred at a given bit position, the comparator 205 may output alogic 1 (e.g., high voltage) to the corresponding integrator 210 bitposition; through internal feedback, that logic 1 will continue to bestored in a latch 215, as a bit fail (BF) flag or signal. Moreparticularly, once a bit fail flag is set, it is never overwritten (andunset) as passing for the remainder of the testing. As a consequence,the existence of any error is maintained for all data words of alltesting cycles and all testing patterns, individually for each bitposition, preserving knowledge of bit position failure (instead ofprematurely OR'ing the comparison results to generate a single bit failflag for the entire memory array 110). This preservation of information,for each bit position, provides the capability for the moresophisticated testing of the present invention. The integrator 210 maybe implemented equivalently with a variety of circuits, such as anaccumulator, adder, register, etc.

As testing is occurring, error information may be provided from theintegrator 210 to the test control circuit 220 on a continuous basis, orfollowing completion of testing. Generally, when error information isprovided on a continuous basis, if there are a sufficient number oferrors across the bit positions, the memory array 110 may be failedprior to test completion. Passing the memory array 110, however, shouldonly occur following completion of all testing, i.e., when all testpatterns have been stored, compared, and test results accumulated.

In exemplary embodiments, test control circuit 220 is implemented usingcombinatorial or conditional logic with an acceptable (or unacceptable)error level based upon the correction capability of the selected errorcorrection coding utilized in the memory array 110. For example, in anexemplary embodiment, either no errors or at most one error in one bitposition are allowable, as within the correction capability ofapplicable error correction coding, such that the memory array 110 isfailed if there are errors in two or more bit positions. For thisexample, assuming the integrator 210 is initially set to zero and anerror is indicated by a logic 1, the test control circuit 220 may beimplemented by circuitry which performs the following combinatorial andconditional functions using a CASE statement:

CASE(BF) 1000....000 = Pass 0100....000 = Pass 0010....000 = Pass . . .through 0000....010 = Pass 0000....001 = Pass 0000....000 = Pass ELSE =Fail.Equivalent implementations of test control circuit 220 may be made usingHDL or VERILOG, or by changing the state of the bit fail flag (e.g., apass being designated by a 1, and a fail by a 0). As may be seen fromthe above example, each bit position is examined for any error that mayhave occurred from any testing pattern. A memory array 110 is allowed topass if there are one or no errors for all bit positions, and otherwisefailed, with such pass or fail information provided to a pass/failindicator 230, such as setting a built-in self test (BIST) Pass flag orsignal.

This invention may be implemented in a variety of ways, such as based onan allowable number of bit errors per data word, or an allowable numberof bit errors per a plurality of data words, or an allowable number ofbit errors per entire memory array 110. In the exemplary embodiment, asa memory array 110 generally provides the capacity for a plurality ofdata words per row 115, and by allowing at most one bit error per dataword, it is conceivable that the fine-grained testing of the presentinvention will allow a memory array 110 to pass having a plurality ofbit errors which are, nonetheless, correctable as within the errorcorrecting capability of the selected error correction code. As aconsequence, manufacturing yield is generally increased as a result ofapplication of the present invention.

It will be apparent to those of skill in the art that other conditionallogic may be implemented based on the correction capability of theselected error correction code, e.g., 2 or more errors may be allowed, 3or more errors may be allowed, and so on. In addition, while more errorsmay be allowable, there may be other limitations which constrain theallowable number of errors, such as test control circuit 220 complexity,IC specifications, etc. Numerous other implementations of test controlcircuit 220 will also be apparent to those of skill in the art, with allsuch implementations considered equivalent and within the scope of thepresent invention.

In addition, while the self test circuit 200 is illustrated in FIG. 3 asdiscrete logic or circuit blocks, it should be understood that the selftest circuit 200 may be embodied or implemented in a wide variety ofways which perform the functionality discussed above. For example, theself test circuit 200 may be implemented as or part of a processor,microprocessor, digital signal processor, controller, field programmablegate array, adaptive computing circuit, and so on (individually andcollectively referred to as a “processor”), particularly when includedwith embedded memory. For example, the methodology of the invention maybe embodied as a set of program instructions or configurations executedby a processor having an embedded memory for self-testing of theembedded memory.

FIG. 4 (or FIG. 4) is a flow diagram of an exemplary method 300 ofmemory testing in accordance with the teachings of the invention, andprovides a useful summary. It should be understood that many of theloops illustrated in method 300 may be performed in parallel, such asthrough implementation of the system 100 and self-test circuit 200.

Beginning with start step 305, the method stores input test data inmemory, step 310, and reads stored test data from memory, step 315. Themethod then performs a bit-wise comparison of the stored test data withthe input test data, step 320. When the stored test data bit is not thesame as the input test data bit (i.e., a bit error has occurred), step325, the method provides and maintains an indication of failure for thecorresponding bit position, step 330, such as by setting and notoverwriting a bit fail flag. Following step 330, or when the stored testdata bit is the same as the input test data bit (i.e., a bit error hasnot occurred) in step 325, the method determines whether there areadditional bit positions to be tested, step 335, and if so, returns tostep 320 for additional comparisons. (As indicated above, steps 320,325, 330 and 335 are generally implemented in parallel.) Otherwise, whenthere are no additional bit positions to be tested, the methoddetermines whether the total number of failures for all bit positions isless than a predetermined error level (as determined by the errorcorrection capability of the ECC or other design specifications), step340. Step 340 may be implemented by summation or accumulation of biterrors for all bit positions, followed by a comparison to apredetermined level, or, for example, by the combinatorial andconditional logic discussed above. Step 340 may also be implementedequivalently, such as by whether the total number of failures for allbit positions is greater than a predetermined error level.

When the total number of failures for all bit positions is not less thanthe predetermined error level in step 340, the memory is designated asfailed, step 345, and the method may end, return step 365.Alternatively, as mentioned above, the comparison and failuredetermination steps 340 and 345 may be performed following completion ofall testing, following step 355. When the total number of failures forall bit positions is less than the predetermined error level in step340, the method determines whether there are additional data words (orrows) to be tested, step 350, and if so, the method returns to step 315,to read the additional data words (or rows) from the memory. When thereare no additional data words (or rows) to be tested in step 350, themethod determines whether there are additional test patterns to beutilized, step 355, and if so, the method returns to step 310, to storetest data in the memory. When there are no additional test patterns instep 355, the memory may be designated as passing, step 360, and thetesting method may end, return step 365.

There are numerous advantages of the present invention. If currentmethods were utilized to screen out memories with two or more defectivebits per data word, BIST test time would be substantially increased. Inorder to exhaustively screen out memories with two or more defects perdata word, while passing those memories with no more than one defect perdata word, considerably greater test pattern complexity would berequired. Such test pattern complexity would be required with prior artmethods because different bits in a data word may require differentconditions to make them fail, such as data polarity and the state oroperation of surrounding cells 105, so that to detect a double bit errorin a single read cycle can require unique conditions to be forced ondifferent bits simultaneously. To reduce the complexity and length ofthe BIST test patterns, various assumptions have been made in the priorart, such as the assumption that all defective memory cells are of the“stuck at” type, and therefore do not test for all possible combinationsof defects known as “nearest neighbor” effects. This prior artmethodology reduces the fault coverage of the BIST test, generatingrisks for the memory user.

In addition, other prior art methods suffer from no direct read/writeaccess to the storage locations of the ECC bits (e.g., ECC parity bits),making it considerably more difficult to place precise test datapatterns in the ECC bit cells to test for nearest neighbor effects. Thepresent invention does not have this limitation, as all bits of thememory array 110 are tested directly, prior to any error correction.

The various embodiments of the present invention also significantlyreduce BIST testing time, as fewer bit error testing patterns arerequired, while nonetheless providing complete error testing. Moreparticularly, bit-pair error patterns (in which bit-pairs are examinedto determine simultaneous failure) are not required, because the presentinvention examines each bit position independently and maintains anyfailure information on a per-bit basis. Where “N” is the number of bitsper data word, and “x” is the testing time for one-bit errors, such bitpair testing requires Nx of testing time. In contrast, the presentinvention provides this level of testing, but reducing the test time by1/N, which is a huge test time savings.

In summary, the various embodiments of the invention integrate overtime, for individual bit defects, the failure results of BIST testpatterns that are able to efficiently detect stuck at faults and nearestneighbor effect faults. Then, for example, by demonstrating that at mostone data bit position ever fails for all memory defects in the memoryinstance, it is known that all failures are then ECC correctable forthat corresponding capability of the selected ECC. This avoids thelonger testing time or incomplete fault coverage of the multiple biterror detecting “exhaustive pattern” method described previously. Thepresent invention works efficiently, as empirical results have indicatedthat as many as 90% of defective memories have defects affecting onlyone data bit position of the memory instance. The soft error correctionefficiency of the ECC is negligibly compromised because only a verysmall percentage of data words per instance of a memory array 110 areobserved to have “hard” errors which utilize the ECC correctioncapability, allowing the ECC to be kept in reserve to correct for “soft”errors.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the novel concept of the invention. It is to be understood thatno limitation with respect to the specific methods and apparatusillustrated herein is intended or should be inferred. It is, of course,intended to cover by the appended claims all such modifications as fallwithin the scope of the claims.

1. A self-test apparatus for a memory, the memory adapted to store inputtest data and output stored test data during a plurality of memory readand write test operations, the apparatus comprising: a comparatorcoupled to receive the input test data and the stored test data, thecomparator adapted to compare the input test data and the stored testdata for a plurality of bit positions, and to provide a correspondingerror signal when the stored test data is not identical to the inputtest data for each bit position of the plurality of bit positions; anintegrator coupled to the comparator to receive the corresponding errorsignal, the integrator adapted to maintain the corresponding errorsignal for each bit position during the plurality of test operations;and a test control circuit coupled to the integrator, the test controlcircuit adapted to provide a fail signal when a predetermined level ofcorresponding error signals have been provided for the plurality of bitpositions.
 2. The memory self-test apparatus of claim 1, wherein theintegrator comprises a plurality of latches, each latch of the pluralityof latches corresponding to a bit position of the plurality of bitpositions.
 3. The memory self-test apparatus of claim 2, wherein eachlatch has an output and an input, with the latch output being coupled tothe latch input to maintain the corresponding error signal during theplurality of test operations.
 4. The memory self-test apparatus of claim1, wherein the predetermined level of corresponding error signals isdetermined by an error correction capability of a selected errorcorrection code.
 5. The memory self-test apparatus of claim 1, whereinthe predetermined level of corresponding error signals is at least twocorresponding error signals.
 6. The memory self-test apparatus of claim1, wherein the test control circuit is further adapted to provide a passsignal when there is at most one corresponding error signal.
 7. Thememory self-test apparatus of claim 1, further comprising: a test datagenerator coupled to the comparator and coupled to the memory to providethe input test data.
 8. The memory self-test apparatus of claim 7,wherein the test data generator is adapted to generate a plurality oftest patterns corresponding to the plurality of test operations, eachtest pattern of the plurality of test patterns adapted to detectsingle-bit errors.
 9. The memory self-test apparatus of claim 1, whereinthe plurality of bit positions correspond to a data word having aplurality of data bits and a plurality of error correction bits.
 10. Thememory self-test apparatus of claim 1, wherein the plurality of bitpositions correspond to a plurality of data words, wherein each dataword of the plurality of data words has a plurality of data bits and aplurality of error correction bits.
 11. The memory self-test apparatusof claim 1, wherein the plurality of bit positions correspond to aplurality of columns of the memory.
 12. The memory self test apparatusof claim 1, wherein the test control circuit is implemented usingcombinatorial and conditional logic gates which provide the fail signalwhen there are at least two corresponding error signals.
 13. The memoryself test apparatus of claim 12, wherein the combinatorial andconditional logic gates are specified by a CASE statement which providesa pass signal when no corresponding error signal has occurred for allbit positions or when at most one corresponding error signal hasoccurred for all bit positions, and otherwise provides the fail signal.14. The memory self-test apparatus of claim 1, wherein the plurality ofmemory read and write test operations utilize a plurality of single-biterror test patterns.
 15. The memory self-test apparatus of claim 1,wherein the memory self test apparatus is integrated within a memoryintegrated circuit.
 16. The memory self-test apparatus of claim 1,wherein the memory self-test apparatus is integrated with an embeddedmemory of an integrated circuit having a plurality of additional,non-memory functions.
 17. The memory self-test apparatus of claim 1,wherein the memory self-test apparatus is embodied as a processor. 18.The memory self-test apparatus of claim 1, wherein the fail signal is afirst state of a memory pass/fail indicator, and wherein a memory passis a second state of the memory pass/fail indicator.
 19. A memorysystem, comprising: a memory array adapted to store input test data andoutput stored test data during a plurality of memory read and write testoperations; a comparator coupled to the memory array to receive theinput test data and the stored test data, the comparator adapted tocompare the input test data and the stored test data for a plurality ofbit positions, and to provide a corresponding error signal when thestored test data is not identical to the input test data for each bitposition of the plurality of bit positions; an integrator coupled to thecomparator to receive the corresponding error signal, the integratoradapted to maintain the corresponding error signal for each bit positionduring the plurality of test operations; and a test control circuitcoupled to the integrator, the test control circuit adapted to provide afail signal when a predetermined level of corresponding error signalshave been provided for the plurality of bit positions.
 20. The memorysystem of claim 19, wherein the integrator comprises a plurality oflatches, each latch of the plurality of latches corresponding to a bitposition of the plurality of bit positions, wherein each latch has anoutput coupled to an input of the latch to maintain the correspondingerror signal during the plurality of test operations.
 21. The memorysystem of claim 19, wherein the predetermined level of correspondingerror signals is determined by an error correction capability of aselected error correction code.
 22. The memory system of claim 19,wherein the predetermined level of corresponding error signals is atleast two corresponding error signals.
 23. The memory system of claim19, wherein the test control circuit is further adapted to provide apass signal when there is at most one corresponding error signal,wherein the fail signal is a first state of a memory pass/failindicator, and wherein a pass signal is a second state of the memorypass/fail indicator.
 24. The memory system of claim 19, furthercomprising: a test data generator coupled to the comparator and to thememory array to provide the input test data, the test data generatoradapted to generate a plurality of test patterns corresponding to theplurality of test operations, each test pattern of the plurality of testpatterns adapted to detect single-bit errors.
 25. The memory system ofclaim 19, wherein the plurality of bit positions correspond to one ormore data words, each data word having a plurality of data bits and aplurality of error correction bits.
 26. The memory system of claim 19,wherein the plurality of bit positions correspond to a plurality ofcolumns of the memory array.
 27. A method of testing a memory array, thememory array adapted to store input test data and output stored testdata during a plurality of memory read and write test operations, themethod comprising: comparing the input test data and the stored testdata for a plurality of bit positions; providing a corresponding errorsignal when the stored test data is not identical to the input test datafor each bit position of the plurality of bit positions; maintaining thecorresponding error signal for each bit position during the plurality oftest operations; and providing a fail signal when a predetermined levelof corresponding error signals have been provided for the plurality ofbit positions.
 28. The method of testing a memory array of claim 27,wherein the predetermined level of corresponding error signals isdetermined by an error correction capability of a selected errorcorrection code.
 29. The method of testing a memory array of claim 27,wherein the predetermined level of corresponding error signals is atleast two corresponding error signals.
 30. The method of testing amemory array of claim 27, further comprising: providing a pass signalwhen there is at most one corresponding error signal.
 31. The method oftesting a memory array of claim 27, further comprising: generating aplurality of test patterns corresponding to the plurality of testoperations, each test pattern of the plurality of test patterns adaptedto detect single-bit errors.
 32. The method of testing a memory array ofclaim 27, wherein the plurality of bit positions correspond to a one ormore data words of a plurality of data words, wherein each data word ofthe plurality of data words has a plurality of data bits and a pluralityof error correction bits.
 33. The method of testing a memory array ofclaim 27, further comprising: providing the fail signal as a first stateof a memory pass/fail indicator; and providing a pass signal as a secondstate of the memory pass/fail indicator.
 34. A memory system,comprising: a memory array adapted to store input test data and outputstored test data during a plurality of memory read and write testoperations; a test data generator coupled to the memory array to providethe input test data, the test data generator adapted to generate aplurality of test patterns corresponding to the plurality of testoperations, each test pattern of the plurality of test patterns adaptedto detect single-bit errors; a comparator coupled to the test datagenerator to receive the input test data and coupled to the memory arrayto receive the stored test data, the comparator adapted to compare theinput test data and the stored test data for a plurality of bitpositions, and to provide a corresponding error signal when the storedtest data is not identical to the input test data for each bit positionof the plurality of bit positions, wherein the plurality of bitpositions correspond to one or more data words, each data word having aplurality of data bits and a plurality of error correction bits; anintegrator coupled to the comparator to receive the corresponding errorsignal, the integrator adapted to maintain the corresponding errorsignal for each bit position during the plurality of test operations;and a test control circuit coupled to the integrator, the test controlcircuit adapted to provide a fail signal when a predetermined level ofcorresponding error signals have been provided for the plurality of bitpositions, the predetermined level of corresponding error signalscorresponding to an error correction capability of a selected errorcorrection code.